I'm up to speed on particle effects, but I', not sure they can efficiently do this effect: In films you see a car driving on a very long straight road in the desert with a trail of airbourne dust trailing them for 100s of metres - like a localised fog over the road. Now particles can do this but the very long lifetime means you'd need a very large number of them. Should I be looking at some type of volumetric fog (like having nebulae you can fly through in space games)- and if so can you suggest a good resource - or something else?

You won't need that many particles - just one large-ish fog sprite for each point along the trail. I can't imagine that you'd need more than a couple of hundred at the very most.

You do, however, get hella overdraw and burn fillrate most quickly. I doubt there's any way to avoid that, though, unless you do something *very* swanky with shaders.

investigate billboarding, using transparent dust textures and animating them and make them always face the user etc

ace

I would use a particle system with large particles. Have them scale up in size as they get older and have the particles animated, it should look good enough. With the particle system method you have a lot more control and it should be faster. With the particles you could add a wind into the level and have the particles blow of to the side and such.

For such specific case you could create a spetialized particle system object, derived from a generic system.

You want is a volumetric line using particles. This particleLine object takes parameters for both endpoints, and interpolates between them with a dynamic number of particles.

The number of particle can vary depending on how far apart the two endpoints are, in screen space. A line vieweed form it`s side gets more particles so it won`t have gaps, and a line that points nearly directly at the screen gets less particles, because at such angle only a few are needed to convey the effect. This also avoids the massive overdrawn when looking down the line.

Then all you need is to blend in/out particles that are being removed/added, and with some tweaking you`ll have a nice volumetric line that won`t eat your fillrate when viewed from certain angles.

Using standard particles which expand and become more transparent is the obvius approach and would look good from an isometric camera angle, but what about a car following another at a couple of hundred metres - wouldn't they see artifacts?